A radiation tomograph for imaging a radioactive drug distribution is disposed in a medical institution. A description will be given of a specific configuration of such a radiation tomograph. A conventional radiation tomograph includes a detector ring in which radiation detectors for detecting radiation are arranged in an annular shape. This detector ring detects a pair of radiation rays (an annihilation radiation pair) in opposite directions irradiated from a radioactive drug in a subject (for example, see Patent Document 1).
A radiation tomograph for head inspection is present as a type of such a radiation tomograph. This head inspection imaging apparatus will be described in detail. FIG. 22 is a diagram for description of a conventional head inspection imaging apparatus. In the conventional head inspection imaging apparatus, a head of a subject is introduced into a detector ring 62 at the time of inspection. In this state, the detector ring 62 detects an annihilation radiation pair irradiated from the subject.
The detector ring 62 specifies a source of the annihilation radiation pair emitted from the head, and an image indicating a radioactive drug distribution is generated based on this position information. The radioactive drug has a property of gathering at a site at which amyloid accumulates, and thus dementia may be diagnosed when a radioactive drug distribution map is diagnosed.
Such a radiation tomograph has a problem that a generated image is unclear. Absorption of radiation in the head is a cause of disturbance of this image. The annihilation radiation pair generated in the head penetrates the head and is incident on the detector ring 62. The head has a characteristic of absorbing radiation to some extent. This absorption characteristic differs depending on the part of the head. Therefore, when a radioactive drug distribution image is attempted to be generated, a radiation absorption characteristic in the subject becomes uneven and is superimposed on the image.
Therefore, in the conventional radiation tomograph, an absorption characteristic (transmission data) of radiation in the head is measured, and a detection result of the detector ring 62 is corrected based on a result thereof (for example, see Patent Document 1). Such correction operation is referred to as an absorption correction. FIG. 23 illustrates a radiation tomograph having an absorption correction function. Transmission data is measured by generating radiation from a radiation source located outside a body of the subject. This detector ring 63 for acquisition of transmission data is different from the detector ring 62 for detection of an annihilation radiation pair emitted from the head. The mutual detector rings 62 and 63 are arranged adjacent to each other to use a central axis in common. A ring-shaped radiation source is disposed inside the detector ring 63.
Radiation generated from the ring-shaped radiation source is detected by the detector ring 63 for acquisition of transmission data across the head of the subject. When such radiation is detected, it is possible to obtain transmission data on the head. This transmission data represents unevenness of the absorption characteristic of radiation superimposed on the radioactive drug distribution image. Therefore, absorption correction can be performed based on the transmission data.
As a method of acquiring transmission data, in addition to a method described with reference to FIG. 23, one using a CT apparatus and an MRI apparatus in place of the detector ring 63 for acquisition of transmission data has been conceived.
Patent Document 1: JP-A-2007-086089